1. Field of the Invention
The invention relates generally to disk drives. More specifically, the invention relates to detection of mechanical shocks on disk drives.
2. Description of the Prior Art
A huge market exists for hard disk drives for mass-market host computer systems such as servers, desktop computers, and laptop computers. To be competitive in this market, a hard disk drive must be relatively inexpensive, and must accordingly embody a design that is adapted for low-cost mass production. In addition, it must provide substantial capacity, rapid access to data, and reliable performance. Numerous manufacturers compete in this huge market and collectively conduct substantial research and development to design and develop cost innovative hard disk drives to meet increasingly demanding customer requirements.
Each of numerous contemporary mass-market hard disk drive models provides relatively large capacity, often in excess of 1 gigabyte per drive. Nevertheless, there exists substantial competitive pressure to develop mass-market hard disk drives having even higher capacities. Another requirement to be competitive in this market is that the hard disk drive must conform to a selected standard exterior size and shape often referred to as a xe2x80x9cform factor.xe2x80x9d Generally, capacity is desirably increased without increasing the form factor or the form factor is reduced without decreasing capacity.
Satisfying these competing constraints of low-cost, small size, and high capacity requires a design that provides high format efficiency and high areal storage density. Format efficiency relates to the percentage of available area that is available for storing user data rather than being consumed by control data, gaps, etc. A real storage density relates to the amount of data storage capacity per unit of area on the recording surfaces of the disks. The available areal density may be determined from the product of the track density measured radially and the linear bit density measured along the tracks.
The available track density depends on numerous factors including the performance capability of a servo system in the hard disk drive which, among other things, provides for track following, i.e., maintaining alignment of a reading or writing transducer with respect to the centerline of a desired track. One type of servo system, sometimes referred to as an xe2x80x9cembedded servoxe2x80x9d employs servo data on the same disk surface that stores user data to provide signals employed in the operation of the servo system. An embedded servo format for the disk surface has the basic characteristic of a plurality of radially-extending servo-data regions (sometimes referred to as xe2x80x9cservo wedgesxe2x80x9d) and an interspersed plurality of radially-extending user-data regions. Each user-data region has a plurality of user-data track segments, and each servo-data region has a plurality of servo-data track segments.
In accord with another element of an embedded servo format, the servo data include track-identification data used during track-seeking operations, and burst data used during track-following operations. While data are being read in operation of an embedded servo hard disk drive, a transducer head produces a time-multiplexed analog read signal that during a revolution of the disk represents servo data during each of a first set of time intervals; and represents user data during each of a second set of time intervals.
The servo system moves the transducer head toward a desired track during a coarse xe2x80x9cseekxe2x80x9d mode using the track ID field as a control input. Once the transducer head is generally over the desired track, the servo system uses the servo bursts to keep the transducer head over that track in a fine xe2x80x9ctrack followxe2x80x9d mode.
The track has a track pitch corresponding to 1/tpi, where tpi represents tracks per inch (track density). In the fine track follow mode, the servo bursts provide position error information representing the displacement of the transducer head relative to a centerline on the track. Servo electronics process the position error information to determine if the transducer head is either on-track or off-track. If the position error information indicates the transducer head is displaced more than a selected percentage (such as 10%) of the track pitch from the centerline, the servo electronics indicate an off-track condition and inhibits the transducer head from writing data to prevent overwriting an adjacent track. If the position error information indicates the transducer head is within the selected percentage of the track pitch from the centerline, the servo electronics indicate an on-track condition and the transducer head is permitted to write data in the user-data region.
A mechanical shock exerted on the disk drive can cause relative movement (rapid displacement) between the transducer head and track that exceeds the selected percentage of the track pitch from the centerline while the transducer head is writing data in the user-data region. Such a mechanical shock may cause the transducer head to overwrite an adjacent track. For example, the transducer head reads the servo data in the servo-data track segment and indicates an on-track condition. However, a mechanical shock exerted on the disk drive causes strain in the head disk assembly that propagates through the head disk assembly to cause relative movement (displacement) between the transducer head and the track that exceeds the selected percentage of the track pitch while the transducer head is writing data on the disk.
The mechanical shock can be produced from internal or external forces that act on the disk drive. For example, spindle motor vibration can produce an internal force on the disk drive. The environment in which the disk drive is operating can produce an external force on the disk drive. Both the external force and the internal force cause a mechanical shock on the disk drive that results in strain in the head disk assembly. Strain is a function of mechanical shock on the disk drive. The greater the mechanical shock (force on the disk drive), the greater the strain in the head disk assembly.
It is known to mount an accelerometer having piezoelectric material on a printed circuit board assembly that is mounted on the head disk assembly for sensing the mechanical shock exerted on the disk drive while the transducer head is writing data in a user-data track segment on the disk. A beam accelerometer can detect linear shocks along a single axis (unidirectional). Accordingly, more than one beam accelerometer is required for detecting linear shocks in the x,y,z axis, and rotational shocks in the planes corresponding to the x,y,z axis.
U.S. Pat. No. 5,333,138 (the xe2x80x9cRichards Patentxe2x80x9d) discloses a mechanical shock sensor having single-beam and dual-beam cantilever beam accelerometers for measuring acceleration due to mechanical shock exerted on the disk drive. U.S. Pat. No. 5,235,472 (the xe2x80x9cSmith Patentxe2x80x9d) discloses a mechanical shock sensor having piezoelectric material (to form an accelerometer) mounted on a printed circuit board in the disk drive to measure acceleration due to mechanical shock exerted on the disk drive.
The mechanical shock exerted on the disk drive propagates through the head disk assembly and mounting connections before reaching the accelerometer mounted on the printed circuit board assembly. The propagating shock wave causes strain in the beam accelerometer and the piezoelectric material generates a signal representing the detected mechanical shock exerted on the disk drive. If the detected mechanical shock exceeds a threshold, the transducer head is inhibited from writing data. However, a problem with mounting accelerometers on the printed circuited board assembly involves the delays associated with the shock wave reaching (propagating to) the printed circuit board assembly. Because the printed circuit board assembly is mounted to and is not as stiff as the head disk assembly, the printed circuit board assembly attenuates high frequency components of the shock wave. Furthermore, the printed circuit board assembly increases the time period for the shock wave to propagate to the accelerometer. Because of this delay, the shock may cause an off-track condition in the head disk assembly before the accelerometer mounted on the printed circuit board assembly detects the mechanical shock exceeding the threshold. If the mechanical shock is not detected in a timely manner, the transducer head may write on an adjacent track.
U.S. Pat. No. 5,521,772 (the xe2x80x9cLee patentxe2x80x9d) discloses mounting an acceleration rate sensor inside the head disk assembly. The acceleration rate sensor detects rate of change of angular and linear acceleration. The acceleration rate sensor includes spaced-apart piezoelectric transducers (PZT) and seismic mass plates attached to the PZTs. When a mechanical shock is exerted on the disk drive, the shock wave propagates from the head disk assembly to the acceleration rate sensor and causes movement of the PZTs. When the PZTs move, the seismic mass plates induce a stress in the PZTs. The magnitude of the strain depends on the movement of the PZTs. Thus, the mechanical shock on the disk drive is not detected until after the shock wave propagates to the acceleration rate sensor and the seismic mass plates induce strain in the PZTs. This can increase the time period for detecting the mechanical shock. Furthermore, purchasing and mounting the acceleration rate sensor inside the head disk assembly can add to the manufacturing cost.
There is a need for a cost effective technique that reduces the time for detecting and responding to a mechanical shock exerted on the disk drive.
The invention can be regarded as a disk drive being subject to a mechanical shock during a write operation in the disk drive. The disk drive includes a head disk assembly having a housing including a first surface area, and a strain transducer for producing a strain signal representing strain in the first surface area due to the mechanical shock. The strain transducer includes a first electrode, a second electrode, and a volume of piezosensitive material defining a second surface area. The piezosensitive material is disposed between the first electrode and the second electrode. The first surface area overlaps the second surface area. The strain transducer further includes means for securing the first electrode and the piezosensitive material to the first surface area such that the strain in the first surface area is replicated in the piezosensitive material. The disk drive includes means responsive to the strain signal for controlling the write operation in the disk drive.